AND DISTRIBUTION OF THE FIROLIDA. 257 
especially in Carinaria, the outer tunie is rough and raised into pointed papille or 
tubercles, the interspaces being often covered with the above-mentioned cellular patches 
(Plate XLIV. figs. 14, 15,). The nature of these we can only conjecture, and suppose 
that the cellules are glands for secreting a slimy fluid like that which covers many 
Gasteropods, both shelled and shell-less, e. g. Helix and Limar; while the papillæ may 
mark minute orifices opening into the aquiferous canals (to be presently spoken of), 
and thus correspond to the membranous tubes and disk-pores of Asterias and Echinus. 
Similar tubercles, said to be of this nature, are frequently found in Salpa. 
The inner or muscular coat is a stronger and more complete investment, which aids 
more than the above in preserving the shape of the animal. It extends from one ex- 
tremity to the other, enclosing the buccal and branchial nuclei, alimentary canal, nervous 
system, aorta, venous sinuses of the return circulation, and other organs, and consists of 
two layers. 
a. The longitudinal or outer is exceedingly thin, and often incomplete along the 
median line both of the back and belly (Plate XLIV. figs. 9, 10), especially the latter, 
where a narrow transparent raphe exists, only sparingly supplied with muscular fibres, 
most evident between the eyes and tail (figs. 9 & 10, c)—and least marked between the 
eyes and mouth (fig. 9,0), where the fibres, both longitudinal and transverse, are more 
pronounced than in any other part of the animal. On either side of the median line, 
both above and below, this layer is thickened and strengthened so as to form a band 
(figs. 9 & 10, b), most obvious on the dorsal surface between the eyes and tail (fig. 9, b). 
A few decussating fibres occasionally cross the raphe. As a whole, the longitudinal 
are more numerous, better defined, and, as a layer, stronger than the transverse or 
inner fibres. 
b. The circular, transverse, or inner stratum (Plate XLITI. figs. 9, 10) surrounds the 
body in its entire length, and is most strongly developed between the eyes and mouth 
(figs. 9 & 10, e) ; but behind those it is often interrupted by the raphe, which only a limited 
number cross. A considerable part of the animal along the median line is thus free from 
muscular fibres, the tunie forming the only covering. Between the eyes the transverse 
fibres are thicker and more highly developed than elsewhere (figs. 9 & 10, f), and form 
strong bands connecting the two eyes, and so well defined, especially on the ventral sur- 
face (fig. 10, f), as to resemble distinct interocular muscles ; while in the same locality both 
longitudinal and transverse fibres, interrupted by the orbital orifice, sweep round the base 
of both eyes and become so thickened as to be somewhat like an imperfect orbicular 
muscle (fig. 9,7). This peculiar arrangement and increased muscular development in the 
neighbourhood of the eyes unquestionably serves a double purpose—first, that of protect- 
ing them and the optic nerves, cephalic ganglion, and other delicately organized structures 
in this vicinity, and, second, that of altering the direction of the eyes and the axis of 
vision. The transverse fibres are also well marked around and for some distance behind 
the oral orifice; and from the posterior edge of this on the ventral aspect another well- 
defined patch (fig. 10,9) runs back, gradually narrowing, till it blends with the inter- 
ocular muscle. Here the longitudinal muscular fibres are also well developed, and run 
back to amalgamate with those along either side of the raphe _ ms wide 
